Introduces the render module to contain different renderers for menus,
and order lists. Makes render_tree_menu the default renderer can deal
with unbalanced trees. Minor refactor in views to introduce
init_session and use the menu renderer configured in
settings.py. two_lvl render is still found in render/menu.py but may
not be fully functional as it is inferior to renter_tree_menu and
therefore not too relevant to keep around.

Centralizes the conversion str->ObjectId in mongo.py for better
readability and maintainability

Improves overall logging and documentation